devicestate: keep log from install-mode on installed system

overlord/devicestate/devicestate_install_mode_test.go

@@ -89,6 +89,9 @@ func (s *deviceMgrInstallModeSuite) SetUpTest(c *C) {
 	s.state.Lock()
 	defer s.state.Unlock()
 	s.state.Set("seeded", true)
+
+	fakeJournalctl := testutil.MockCommand(c, "journalctl", "")
+	s.AddCleanup(fakeJournalctl.Restore)
 }
 
 const (

overlord/devicestate/handlers_install.go

@@ -20,8 +20,10 @@
 package devicestate
 
 import (
+	"compress/gzip"
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 
 	"gopkg.in/tomb.v2"
@@ -90,6 +92,37 @@ func writeModel(model *asserts.Model, where string) error {
 	return asserts.NewEncoder(f).Encode(model)
 }
 
+func writeLogs(rootdir string) error {
+	// XXX: would be great to use native journal format but it's tied
+	//      to machine-id, we could journal -o export but there
+	//      is no systemd-journal-remote on core{,18,20}
+	//
+	// XXX: or only log if persistent journal is enabled?
+	logPath := filepath.Join(rootdir, "var/log/install-mode.log.gz")
+	if err := os.MkdirAll(filepath.Dir(logPath), 0755); err != nil {
+		return err
+	}
+
+	f, err := os.Create(logPath)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	gz := gzip.NewWriter(f)
+	defer gz.Close()
+
+	cmd := exec.Command("journalctl", "-b", "0")
+	cmd.Stdout = gz
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("cannot collect journal output: %v", err)
+	}
+	if err := gz.Flush(); err != nil {
+		return fmt.Errorf("cannot flush compressed log output: %v", err)
+	}
+	return nil
+}
+
 func (m *DeviceManager) doSetupRunSystem(t *state.Task, _ *tomb.Tomb) error {
 	st := t.State()
 	st.Lock()
@@ -237,6 +270,11 @@ func (m *DeviceManager) doSetupRunSystem(t *state.Task, _ *tomb.Tomb) error {
 		return fmt.Errorf("cannot make run system bootable: %v", err)
 	}
 
+	// store install-mode log into ubuntu-data partition
+	if err := writeLogs(boot.InstallHostWritableDir); err != nil {
+		logger.Noticef("cannot write installation log: %v", err)
+	}
+
 	// request a restart as the last action after a successful install
 	logger.Noticef("request system restart")
 	st.RequestRestart(state.RestartSystemNow)

tests/nested/core20/basic/task.yaml

@@ -48,3 +48,8 @@ execute: |
     nested_exec 'cat /var/lib/snapd/save/device/asserts-v0/serial/'"$(nested_model_authority)"'/pc/*/active' >serial.saved
     nested_exec snap model --serial --assertion >serial
     cmp serial serial.saved
+
+    # check that we go the install log after the transition to run mode
+    nested_exec "test -e /var/log/install-mode.log.gz"
+    # transparently verify that the format is gzip
+    nested_exec "zcat /var/log/install-mode.log.gz" | MATCH 'installing a new system'